Oh you are in one of those areas that the parasites just love--not too cold and not too hot. So you should deworm all year round rather than stopping during the winter (strongyle eggs can hatch and become infective even when the temps are in the 40's).
You don't have a huge tapeworm problem in WA, so once a year deworming with praziquantel or double dose pyrantel would be appropriate though you can do it twice a year if you wish.
With only 1 other horse in the pasture, you don't have as much contamination as you would with a whole herd in there and you've got a better chance of keeping up with picking the pasture every couple of days--this would cut down on the parasite load tremendously.
If you pick the pastures and deworm on a regular schedule for a year, you could cut the parasites burden on the pasture enough that you could go to fecal egg counts every few months and deworm only when the parasite load in your horse got above 125 eggs per sample.
So for a year you could either deworm every 3 months with moxidectin or deworming on a rotating schedule using pyrantel, then deworming in 4 weeks with ivermectin, then in 8 weeks with moxidectin then 12 weeks later deworm again. So long as you stick with deworming based upon the egg reapearance period for the drug you last used (4 weeks for pyrantel, 8 for ivermectin, 12 for moxidectin) you don't have to use any particular product the next time around...just do it at the right time and then deworm at the right time after that one. The only 2 times a year you need to use a something specific is spring and fall bots and either ivermectin or moxidectin will work for that.
After a year of this deworming program, have a fecal egg count done (being sure to wait for a couple of weeks after the egg reappearance period of the last drug you used) and see what kind of parasite load your horse has. Because your parasite burden on the pasture will be much lower, you can usually spread out your dewormings quite a bit this way.
Licensed Veterinary Technician